The present invention relates to conversion of vestigial sideband signals to base band, and more particularly to providing spectral compensation for vestigial sideband signals having a variable or poorly regulated intermediate frequency (IF).
In order to conserve bandwidth and reduce power requirements, it is common practice to transmit television signals as vestigial sideband, VSB, signals. VSB signals are produced by modulating a signal with a standard double sideband full carrier modulator and then filtering out a portion of the lower sideband before transmission. This reduces the bandwidth and power required to transmit a signal while providing an easily demodulated signal.
In order to obtain an accurate base band signal from a VSB signal, spectral compensation is needed. Generally, this compensation is in the form of a filter having a lower cutoff centered on the carrier frequency, usually the IF frequency. It is therefore necessary to know the IF frequency in order to design an appropriate compensation filter.
There are two “standard” IF frequencies used for cable applications. In the United States the standard is 44 MHz, while in Europe the standard is 36 MHz. Each of these requires the use of a different VSB compensation filter. In addition, IF frequency errors of plus or minus 150 KHz occur as a result of RF modulation errors in the transmitter and errors in the local RF to IF tuner. It would be expensive in terms of component cost and board area for an analog filter solution or silicon area for a digital solution to provide a range of compensation filters capable of providing accurate spectral compensation for VSB signals over this range of IF frequencies.
It would be desirable to provide a system for providing spectral compensation for VSB signals with a single compensation filter.